1. Field of the Invention
The present invention relates to a method and an apparatus for identifying the type of an optical recording medium, and more particularly, to a method and an apparatus for identifying the type of an optical recording medium using a predetermined optical system.
2. Description of the Related Art
In general, optical recording media, i.e., optical disks, are divided into a ROM-type (read only) optical disk, a write once optical disk, and a re-writable optical disk, depending on whether and how many times they can be recorded.
Such optical disks are generally divided into a compact disk (CD), a digital versatile disk (DVD), and a next-generation DVD depending on their storage capacities. A CD has a storage capacity of 650 MB, a DVD has a storage capacity of 4.7 GB, and a next-generation DVD has a larger storage capacity compared with that of a DVD.
Optical disks have different sizes depending on their storage capacities. A CD and a DVD have the same diameter, which is 120 mm, but have different thicknesses. A CD has a thickness of 1.2 mm, and a DVD has a thickness of 0.6 mm. A next-generation DVD is known to have a thickness of about 0.1 mm. A laser beam of 0.4 numerical aperture (NA) and a wavelength of 780 nm is generally used for CDs, and a light beam of 0.6 NA and a wavelength of 650 nm is used for DVDs. A laser beam of 0.85 NA, which belongs to a blue wavelength band, is known to be used for next-generation DVDs.
FIG. 1 is a diagram illustrating the thickness of a CD compared with that of a DVD. Referring to FIG. 1, as described above, while a CD has a thickness DC of about 1.2 mm, a DVD has a thickness DD of about 0.6 mm. In other words, a distance between a top surface 31 and a writable surface 35 of the CD is about two times as large as a distance between the top surface 31 and a writable surface 33 of the DVD. Accordingly, as shown in FIG. 2, there exists a difference between a time period DTD between a surface reflected signal SS and a writable-surface reflected signal S1R and a time period DTC between a surface reflected signal SS and a writable-surface reflected signal S2R. 
FIG. 2 is a graph illustrating a conventional method for identifying the type of an optical recording medium. Referring to FIG. 2, the time period DTC of a CD between the surface reflected signal SS and the writable-surface reflected signal S2R is longer than the time period DTD of a DVD between the surface reflected signal SS and the writable-surface reflected signal S1R. Accordingly, in the related art, a certain reference time difference T0 is set in advance to have a median value between DTD and DTC, and then a time period DTp in a predetermined optical recording medium is compared With T0. Here, DTp is defined as a time difference between the time when a signal reflected from a writable surface of an optical disk is detected and the time when a signal reflected from the surface of the optical disk is detected. If DTp is greater than T0, the predetermined optical disk is identified as a CD. On the other hand, if DTp is smaller than T0, the predetermined optical disk is identified as a DVD.
FIG. 3 is a graph showing the variation of a DTp value with respect to a number of times an experiment for obtaining such DTp value has been performed in the case of using the conventional method for identifying the type of an optical recording medium. Referring to FIG. 3, a DT value obtained for a CD or a DVD varies considerably with the number of experiments. In particular, in the case of a CD, a DT value in a fourth experiment is 80s, and a DT value in a fifth experiment is 130s. Thus, there exists a difference of a maximum of 50s between DT values. In addition, in a twelfth experiment, a DT value for a CD and a DT value for a DVD are 80s and 95s, respectively, which are very close to each other, and thus it is not easy to figure out the type of an optical recording medium based on its DT value.
However, it may be difficult to determine the optical disk-type using the conventional method where there is a nonlinear driving region in the optical disk where linearity of an actuator is difficult to maintain. In such a nonlinear driving region, an error is more likely to occur during the calculation of both DTD and DTC, and if that happens, DTD and DTC may approximate to T0. Especially, in the case of identifying the type of a special optical disk, such as a deflected disk, such an error may become even greater, depending on the position and tilting direction of an optical disk.